A building usually has a strong shielding effect on a signal. In an indoor environment such as a shopping mall or an underground parking lot, a communication signal is usually weak, which severely affects normal use of a mobile phone.
Based on the foregoing reason, indoor extension coverage of a base station becomes increasingly important. Various signals that are in different bands and of different operators exist in an indoor area; therefore, in order to prevent repeated investment, currently a multisystem combining platform, also referred to as a point of interface (POI), is put forward. The multisystem combining platform is installed in an indoor area, and can combine different system signals received from an external environment and send combined signals to an indoor mobile terminal.
However, a communication system using a traditional multisystem combining platform has high passive intermodulation (PIM) and poor intra-band isolation; in a scenario in which there are multiple operators and multiple bands and power is high, high PIM easily causes a second-order intermodulation product and a third-order intermodulation product, thereby affecting receiver sensitivity of the base station.